1. Field of the Invention
This invention relates to the removal of mercury from combustion gas streams and more specifically to the use of halogenated carbon materials to reduce the emissions of mercury from coal-fired power plants.
2. Description of the Related Art
It is well known that mercury is both hazardous and poisonous. Consequently, there is frequently a need to remove it from, for example, the air streams around industrial processes, such as at chlor-alkali plants, or from the air in dental offices using amalgams, where people may be directly exposed to mercury vapor. Similarly, there is a need to sequester mercury from natural gas and hydrocarbon streams, where it corrodes processing equipment; from wastewater streams, where its discharge can contaminate ecosystems; and from the hot combustion-gas emissions of waste incinerators, where it is emitted to the environment to methylate and bio-concentrate up the food chain. Each of these gas or liquid streams has different characteristics that make some mercury removal methods effective and appropriate, but others, ineffective and inappropriate. Consequently, over the years, a multitude of approaches have had to be developed for effectively removing mercury species from various streams. These overall approaches include, among others: liquid scrubbing technologies, homogenous gas-phase technologies, metal amalgamation techniques, and processes utilizing various sorbent materials in different application schemes, with adsorbents optionally impregnated with various reaction aids.
A common recent concern is the mercury emitted from coal-fired power plants. It has been estimated, for example, that about 100,000 pounds of mercury are being emitted into the atmosphere annually in the United States from coal-fired power plants. Capturing and isolating this mercury is a very difficult technical problem because the gas volumes to be processed are great, the gas concentrations of the mercury are low, and the gas temperatures are high. Also, many other complicating compounds are present in the flue gas and multiple mercury species have to be sequestered. Even though many mercury control techniques have already been developed, new means for effectively and economically controlling utility mercury emissions are still needed. After a thorough investigation of the prior art on mercury removal from power-plant gas streams, the U.S. Environmental Protection Agency (EPA) concluded in the Executive Summary to its 1998 Utility Hazardous Air Pollutants (HAPs) Report to Congress that:                “Regarding potential methods for reducing mercury emissions, the EPA has not identified any demonstrated add-on control technologies currently in use in the U.S. that effectively remove mercury from utility emissions.” [Page ES-18].        
In the past, activated carbons have demonstrated utility for sequestering mercury vapors in some applications. When combined with halogen compounds, the mercury sequestration performance of activated carbons can be improved. In particular, the ability of iodine and iodide impregnations to increase the capacity of granular activated carbons in capturing elemental mercury vapor from air at ambient temperatures has long been known. Stock U.S. Pat. No. 1,984,164, for example, teaches the advantages of loading activated carbon with halogens, particularly iodine, to remove mercury from ambient air and Dreibelbis et al. U.S. Pat. No. 3,194,629, of impregnating activated carbon with an iodine-potassium iodide mixture. Revoir et al. U.S. Pat. No. 3,662,523 claims improved results with interhalogens such as ICl and ICl3 on filter elements of activated carbon and Anderson U.S. Pat. No. 3,956,458 recommends the use of an elemental sulfur filter followed by an iodine-impregnated filter. Alternately, to purify hydrogen or vent buildings, deJong et al. U.S. Pat. No. 4,196,173 teaches the benefits of injecting elemental chlorine gas ahead of filters of chlorinated active carbon.
Unfortunately, however, impregnated iodine and iodine compounds are released from carbonaceous sorbents at modestly elevated temperatures. Thus, their use is largely limited to ambient-temperature process streams. As explained by Bansal, Donnet, and Wang in their book CARBON BLACK: SCIENCE AND TECHNOLOGY, 2nd Edition, unlike chlorine and bromine, which chemically react with strongly-held carbon surface compounds, iodine compounds are primarily or only physically adsorbed by carbonaceous materials. Consequently, at the elevated temperatures of combustion gas streams, much of any adsorbed iodine or iodides will be released from these materials. Not only could any captured mercury-iodide species evolve off, but the other impregnated iodine species of the materials could volatilize off and corrode downstream structures.
In addition, the above prior art references contact their gas streams with the sorbents in fixed-bed filters. While applicable for small-scale gas processing, it can be cost-prohibitive to run the extremely large volumes of hot flue gas from a power plant through fixed or moving beds of granular carbon. The energy costs of the pressure drop and the fixed costs of the vessel can be unreasonably high, even if the sorbent costs themselves could be kept manageable.
Rather than using iodine or chlorine impregnating gases directly, dissolved metal halides can be advantageously applied to carbon substrates to promote mercury sequestration. Japanese patents 49053590 through 49053594 to Nippon Soda Co. Ltd. and 49066592 to Sumitomo Chemical Co. report on activated carbons impregnated with various halogen metal salts for mercury removal. In addition, Japanese patent 51003386 recommends activated carbon impregnated with a hydrogen halide salt of a compound with one or more functional groups for mercury sequestration. Similarly, in U.S. Pat. No. 4,500,327 Nishino, Aibe, and Noguchi teach that mercury vapors can be advantageously removed from air, natural gas, and incinerator exhausts by activated carbons impregnated with combinations of sulfur, metal sulfates or nitrates, iodine oxides or oxyacids, and the iodides or bromides of K, Na, or NH4. In U.S. Pat. No. 6,533,842 Maes et al., a cupric chloride impregnated carbon in combination with calcium hydroxide is shown to improve mercury reductions from a gas stream. And finally, in publications such as “In-Flight Capture of Elemental Mercury by a Chlorine-Impregnated Activated Carbon,” Air & Waste Management Association Paper #731 at the 2001 Annual Meeting, Ghorishi et al., discloses the potential benefits using dilute solutions of hydrogen chloride, HCl, as an impregnate.
Unfortunately, the production of halogenated carbons from dissolved metal halides or hydrogen halide salts is laborious and difficult to perform on a large scale. High-quality base carbons are generally used, the impregnates must be dissolved in a solvent, applied evenly to the fine carbon substrates, the solvents removed, and the carbons wetted, washed, dried, delumped, and sometimes post-processed with heating in inert atmospheres. Working with sorbents made from HCl solutions, for example, Ghorichi et al. found that use of special, deionized water and slow, low-temperature drying were required in order to preserve mercury performance improvements. Consequently, while sorbents made from dissolved halide species may perform well, they end up being very expensive. In the recent Utility HAPs Report to Congress, which included a detailed evaluation of the control technologies available for power plant mercury control, the U.S. EPA reported that:                “Sulfur-, iodide-, chloride salt-, and Ca(OH)2-impregnated activated carbons show promise for increasing the mercury removal efficiency, but further testing is needed. [However, t]he cost of these modified carbons can be as much as 20 times higher than that of unmodified AC.” [Page 13-42.]        
These high costs, primarily due to their solution-based manufacture, make them uneconomic for duct-injection use at power plants with electrostatic precipitators (ESPs), because large volumes of sorbents are required and they are ultimately thrown away with the fly ash.
In addition to their high costs, carbons impregnated by dissolved halide salts can have the cations of their salts, such as the heavy metals copper, cadmium, strontium, and zinc of prior-art patents, leach into the groundwater when their resulting fly ashes are landfilled.
The particular advantages of using bromine, rather than iodine or iodides, or chlorine or chlorides, with activated carbons for mercury control have not been previously appreciated. Gaseous bromine and hydrogen bromide have been combined with carbon substrates before, but not to sequester mercury from hot combustion-gas streams. For example, Greinke U.S. Pat. No. 5,372,619 found that bromine-treated carbon can make a superior natural-gas storage medium. In another example, SKC Inc. sells a small tube with hydrobromic acid-treated charcoal to sample air for ethylene oxide. However, with both of these uses it is important that the adsorption targets, natural gas and ethylene oxide, be easily desorbable from the carbon, the exact opposite of what is required in a mercury vapor sequestration application. In a similar vein, in U.S. Pat. No. 6,475,461 Ohsaki describes a process for treating carbon substrates with gaseous bromine or chlorine, but then explicitly desorbs them to achieve his desired product. Seki U.S. Pat. No. 3,961,020; Yoshida and Seki et al. U.S. Pat. No. 4,174,373; and Knoblauch et al. U.S. Pat. No. 5,179,058 impregnate activated carbon with bromine to produce a catalyst for reacting nitrogen oxide with ammonia to form nitrogen and water. In this application too, the bromine of the carbon does not act as a sequestration agent, permanently tying up its target. Rather, it serves as a catalyst, taking part in a repeated series of desired chemical reactions, but not becoming permanently consumed by any of them. Perhaps it is understandable that the tenacity of carbon-bromine-mercury complexes could be overlooked.
Recently, a number of inventive methods have been developed to apply mercury sorbent technologies to the large-scale gas streams of coal combustion for power generation. The U.S. patents of Moller et al. U.S. Pat. No. 4,889,698 and Chang, U.S. Pat. No. 5,505,766, for example, describe the injection of fine powdered activated carbon (PAC) into flue gases at points along their journey through various pollution-control equipment trains. A handful of full-scale power-plant sorbent-injection trials have also recently taken place, including one at Great River Energy's Stanton Station capturing an injected, custom-ground, potassium-iodide-impregnated PAC in a fabric filter. While this material removed significantly more mercury than the plain PACs tested at the site, it cost ten times as much. And only about 15% of coal-fired boilers in the U.S. have such fabric filters, which allow for a high degree of mass transfer as the mercury-laden flue gas through a layer of the sorbent on the fabric filter bags. On the other hand, about 65% of U.S. coal-fired utility boilers have ESPs instead of fabric filters, with no flue gas desulfurization systems. This configuration requires in-flight mercury removal, with some amount of time on the ESP plates parallel to the gas flow. Mercury removal at plants with only an ESP is a most difficult mercury-sequestration situation and an application especially targeted by the current invention.
Accordingly, it is an object of the present invention to provide a sorbent material that can be injected into a hot mercury-containing combustion gas, so that a significant portion of the mercury is adsorbed onto the sorbent and removed from the flue gas with the combustion fly ash.
Further, it is an object of the present invention to provide a flexible, retrofitable mercury-control method that can be applied to a number of combustion gas streams and a wide range of exhaust system equipment configurations.
In addition, it is an object of the present invention to provide a mercury sorbent material that is simple and inexpensive to manufacture and use.
It is also an object of the present invention to provide a mercury sorbent material that causes the adsorbed gas-phase mercury to become essentially permanently-sequestered from future interactions with the environment.